This research program consists of several projects, each dealing with an aspect of the localization and function of gonadotropin receptors in the rat testis. (1) EM autoradiography will be used to trace the fate of 125I-hFSH after it is taken up on the surface of Sertoli cells and of spermatogonia. (2) 125I-hFSH will be perfused through a seminiferous tubule in vivo to determine by LM and EM autoradiography if specific FSH binding takes place on membrane surfaces within the adluminal compartment of the tubule. (3) Sertoli cells and spermatogonia will be isolated by cell separation techniques, and studied to see if the two types of testicular FSH receptors reported in the literature are found respectively on these two cell types. (4) The mechanism of desensitization and other aspects of hCG processing by Leydig cells will be studied in vivo by EM autoradiography, and in short-term culture by fluorescein-labeled hormone as well as by EM autoradiography. (5) The separate fates of the alpha and beta subunits of hCG (one subunit labeled with 125I and the other with 131I) will be followed biochemically in Leydig cells both in vivo and in vitro. (6) The cellular composition of Leydig cell fractions from Metrizamide gradients will be studied by electron microscopy to determine the extent and possible effect of macrophage contamination (there is approximately 1 macrophage for each 3 Leydig cells in the intact testis, and the macrophages may take up hormone into lysosomes during cell isolation procedures).